The present invention relates to a reference voltage generation circuit for generating a reference voltage to selectively and cyclically drive a group of driven elements such as, for example, an array of light emitting elements disposed in an electro-photography printer, an array of heating resistors disposed in a thermal printer, and an array of display units disposed in a display device. The present invention also relates to a drive device including the reference voltage generation circuit; a print head including the drive circuit; and an image forming apparatus including the print head.
In a conventional image forming apparatus such as an electro-photography printer, a plurality of light emitting elements is arranged to form an exposure device. The light emitting element includes an organic electro luminescence element (referred to as an organic EL) and a light emitting thyristor, in addition to a light emitting diode (referred to as LED).
In general, the light emitting element as the driven element exhibits temperature dependence, and a luminescence output thereof tends to decrease with an increase in a temperature. In the conventional image forming apparatus such as the electro-photography printer, when the luminescence output of the light emitting element decreases, a print density varies, thereby causing a printing problem. To this end, it is configured such that a drive current for driving the light emitting element increases, thereby making it possible to compensate the decrease in the luminescence output of the light emitting element due to the increase in the temperature when the light emitting element is driven.
In the configuration, a drive device is provided with a reference voltage Vref, so that the drive current of the light emitting element is set reversely proportional to the reference voltage Vref. Further, the reference voltage Vref is provided with a positive temperature coefficient, so that is it possible to compensate the decrease in the luminescence output according to the temperature. Patent Reference has disclosed a conventional reference voltage generation circuit for generating an output voltage as the reference voltage Vref reversely proportional to the absolute temperature.
Patent Reference: Japanese Patent Publication No. 10-332494
When the drive device, the print head, and the image forming apparatus are provided with the conventional reference voltage generation circuit disclosed in Patent Reference, there have been the following problems.
In the print head, a large number of the light emitting elements are arranged. Accordingly, it is necessary to drive a large number of the light emitting elements. As a result, it is necessary to generate a large power source current for driving the light emitting elements, thereby causing a large variance in a power source voltage. Even when the light emitting elements are driven and the power source voltage drops significantly, it is still necessary to maintain a luminescence output of the light emitting elements at a specific level. To this end, it is necessary to design the drive device and a peripheral circuit thereof such that an influence of the large variance in the power source voltage is minimized.
In the conventional reference voltage generation circuit disclosed in Patent Reference, a pair of bipolar transistors is provided for detecting a temperature. The bipolar transistors have a characteristic in which a difference in voltages between a base and an emitter of the bipolar transistors varies according to a temperature. Accordingly, the conventional reference voltage generation circuit is configured such that the difference in the voltages is output as the reference voltage. When the power voltage decreases, the reference voltage tends to decrease. As a result, the drive current varies, thereby causing a variance in a print density.
In view of the problems described above, an object of the present invention is to provide a reference voltage generation circuit capable of solving the problems of the conventional reference voltage generation circuit. A further object of the present invention is to provide a drive device including the reference voltage generation circuit, a print head including the drive circuit, and an image forming apparatus including the print head.
Further objects and advantages of the invention will be apparent from the following description of the invention.